1. Field of the Invention
The present invention relates to an electroluminescence display device comprising electroluminescence elements and thin film transistors.
2. Description of Prior Art
In recent years, electroluminescence (referred to herein after as xe2x80x9cELxe2x80x9d) display devices comprising EL elements have gained attention as potential replacements for CRTs and LCDS. Research has been directed to the development of EL display devices using, for example, thin film transistors (referred to hereinafter as xe2x80x9cTFTxe2x80x9d) as switching elements to drive the EL elements.
FIG. 1 is a plan view showing a display pixel of an organic EL display device. FIG. 2A shows a cross-sectional view taken along line Axe2x80x94A of FIG. 1 while FIG. 2B shows a cross-sectional view taken along line Bxe2x80x94B of FIG. 1.
As shown in these drawings, a display pixel 20 is formed in a region surrounded by a gate line GL and a data line DL. A first TFT serving as a switching element is disposed near an intersection of those lines. The source of the TFT 1 simultaneously functions as a second capacitor electrode 3 such that, together with a first capacitor electrode 2, it forms a capacitor 8. The source is connected to a gate electrode 15 of a second TFT 4 that drives the organic EL element. The source of the second TFT 4 contacts with an anode 6 of the organic EL element, while the drain of the TFT 4 is connected to a power source line (drive line) VL.
The first capacitor electrode 2, which is made of a material such as chromium, overlaps, over a gate insulating film 7, the second capacitor electrode 3 integral with the source of the first TFT 1. The first capacitor electrode 2 and the second capacitor electrode 3 together store charges with the gate insulating film 7 being interposed therebetween as a dielectric layer. The storage capacitor 8 serves to retain voltage applied to the gate electrodes 15 of the second TFT 4.
The first TFT 1, the switching TFT, will now be described.
First gate electrodes 11 made of refractory metal such as chromium (Cr) or molybdenum (Mo) are formed on a transparent insulator substrate 10 made of quartz glass, non-alkali glass, or a similar material. As shown in FIG. 1, the first gate electrodes 11 are integrally formed with the gate line GL such that a plurality of these electrodes extend from the gate line GL in the vertical direction in parallel with each other. Referring to FIG. 2A, the first capacitor electrode 2 formed in the same process as that of the first pate electrodes 11 is provided to the right side of the first gate electrodes 11. This first capacitor electrode 2, which constitutes the storage capacitor 8, has an enlarged portion between the first TFT 1 and the second TFT 4 as shown in FIG. 1 and is integral with a storage capacitor line CL extending therefrom in the directions.
A first active layer 12 composed of poly-silicon (referred to hereinafter as xe2x80x9cp-Sixe2x80x9d) film is formed on the gate insulating film 7. The first active layer 12 is of a so-called LDD (Lightly Doped Drain) structure. Specifically, low-concentration regions are formed on both sides of the gate. Source and drain regions, which are high-concentration regions, are further disposed on the outboard sides of the low-concentration regions on the first active layer 12, a stopper insulating film 13 made of Si oxidation film is formed so as to prevent ions from entering the first active layer 12.
An interlayer insulating film 14 formed by sequential lamination of a SiO2 film, a SiN film, and a SiO2 film is provided on the entire surface over the gate insulating film 7, the active layer 12, and the stopper insulating film 13. The data line DL which functions as a drain electrode is electrically connected, through a contact hole C1 formed in the interlayer insulating film 14, to the drain in the active layer 12. A planarizing insulating film 18 made, for example, of an insulating organic resin is also formed over the entire surface for planarization.
In EL display devices which are driven by an electric current, the EL layers must have a uniform thickness. Otherwise, current concentration may occur in a portion of the layer having thinner thickness. Thus, a significantly high level of planarity is required at least in portions where the EL elements are to be formed, and therefore the above-described planarizing film 18 made of a material having fluidity prior to hardening is employed.
The second TFT 4 which drives the organic EL element will be described with reference to FIGS. 1 and 2B.
On the insulating substrate 10, second gate electrodes 15 made of the same material as the first gate electrodes 11 are provided, and a second active layer 16 is further formed on the gate insulating film 7. Then, a stopper insulating film 17 is formed on the second active layer 16 in a manner similar to the above-mentioned stopper insulating film 13.
Intrinsic or substantially intrinsic channels are formed in the second active layer 16 above the gate electrodes 15, and source and drain regions are formed on respective sides of these channels by doping p-type impurities, thereby constituting a p-type channel TFT.
The above-described interlayer insulating film 14 is provided on the entire surface over the gate insulating film 7 and the second active layer 16, and the power source line VL is electrically connected, through a contact hole C2 formed in the interlayer insulating film 14, to the drain in the active layer 16. Further, the planarizing film 18 is formed over the entire surface, such that the source is exposed through a contact hole C3 formed in the planarizing film 18 and the interlayer insulating film 14. A transparent electrode made of ITO (Indium Tin Oxide) that contacts the source through this contact hole C3, namely, the anode 6 of the organic EL element 20, is formed on the planarizing insulating film 18.
The organic EL element 20 is formed by laminating, in order, the anode 6, an emissive element layer EM comprising a first hole transport layer 21, a second hole transport layer 22, an emissive layer 23 and an electron transport layer 24, and a cathode 25 made of a magnesium-indium alloy. The cathode 25 is substantially disposed over the entire surface of the organic EL elements.
The principle and operation for light emission of the organic EL element is as follows. Holes injected from the anode 6 and electrons injected from the cathode 25 recombine in the emissive layer 23, to thereby excite organic molecules constituting the emissive layer 23, thereby generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode via the transparent insulator substrate and resultant light emission is observed.
As shown in FIG. 2B, the cathode 25 which drives the organic EL element is formed on the entire surface over the display pixel region as a common electrode, and is electrically connected to a terminal provided at one end of the transparent substrate 10.
The above-described structure, however, suffers from the following disadvantages. Namely, a DC or AC potential (most commonly a DC potential for an organic EL) is externally applied to the cathode 25, so that a current flows between the anode 6 and the cathode 25. Therefore, when the cathode 25 and the lines connected to the cathode 25 have high contact resistance or high line resistance, the bias to be applied to the cathode 25 is lowered, thereby degrading display quality.
The present invention was made in light of the above described disadvantages, and aims to prevent voltage drop caused by line resistance when connecting an external power source to a cathode.
To achieve the foregoing object, according to one aspect of the present invention, there is provided an electroluminescence display device comprising, on an insulating substrate, a display region including a plurality of electroluminescence elements and a group of terminals disposed at an end portion of said substrate for connecting the device externally, wherein each of said plurality of electroluminescence elements comprises a first electrode and a second electrode, of which an upper layer electrode formed in the upper layer is electrically connected to corresponding a plurality of terminals of said group of terminals in the periphery of said display region.
According to another aspect of the present invention, there is provided an electroluminescence display device comprising, on an insulating substrate, a display region including a plurality of electroluminescence elements and a group of terminals disposed at an end portion of said substrate for externally connecting the device, wherein each of said plurality of electroluminescence elements comprises a first electrode and a second electrode. In this aspect, a wide line is extended from an upper layer electrode of said first and second electrodes, which are formed to cover the whole of said display region, this wide line extending toward said group of terminals, lines are extended from corresponding plurality of terminals of said group of terminals, said lines being gathered to form a wide line, said wide line at the terminal side and said wide line at the upper layer side are connected in a wide first contact portion formed between said group of terminals and said display region, and a line extending from said plurality of terminals toward the vicinity of the outer periphery of said display region is further provided, said line being connected with said upper layer electrode in a plurality of second contact portions provided in the vicinity of the outer periphery of said display region.
As described above, according to the present invention, the upper layer electrode (or the cathode) is connected to the lines extending from the terminals in the periphery of the display region. Further, the upper layer electrode and the terminals are also electrically connected between the group of terminals and the display region, as described above. By establishing additional connection between the upper layer electrode and the terminals in the vicinity of the display region, the contact area can be increased substantially, thereby reducing the contact resistance and the line resistance. Also, by utilizing a space around the display region so as to connect the upper layer electrode and the terminals, it is possible to efficiently arrange the lines with low resistance within a limited area.
Further, by employing a structure in which a plurality of terminals (cathode terminals, that is, common electrode terminals) are electrically connected to the upper layer electrode, a wide line space corresponding to the number of allotted terminals can be obtained between the plurality of terminals and the display region. Accordingly, it is easy to form the wide line in this space, so that, as well as in the periphery of the display region, the terminals can contact with the upper layer electrode also by means of this wide line, thereby further reducing the contact resistance.
Accordingly, when the plurality of electroluminescence elements emit light when a current is applied to the emissive element layer by the first and second electrodes in the electroluminescence display device of the present invention, it is possible to prevent a decrease in a current as a result of line resistance. Therefore, it is possible to provide a display device capable of emitting light with a high luminance while consuming less power.
According to another aspect of the present invention, there is provided an electroluminescence display device comprising, on an insulating substrate, a display region including a plurality of electroluminescence elements and a group of terminals disposed at an end portion of said substrate for externally connecting the device, wherein each of said plurality of electroluminescence elements comprises a first electrode and a second electrode, of which an upper layer electrode formed in the upper layer is electrically connected to lines extending from corresponding a plurality of terminals of said group of terminals in contact portions formed in the periphery of said display region, and said contact portion includes a contact intermediate layer composed of a conductive oxide between said upper layer electrode and said lines.
According to still another aspect of the present invention, there is provided an electroluminescence display device comprising, on an insulating substrate, a display region including a plurality of electroluminescence elements and a group of terminals disposed at an end portion of said substrate for externally connecting the device, wherein each of said plurality of electroluminescence elements comprises a first electrode and a second electrode which functions as a cathode, so as to cover the entire surface of said display region, lines extending from corresponding terminals of said group of terminals are disposed in the outer periphery of said display region, and said cathode is connected to said lines in contact portions formed in the outer periphery of said display region, said contact portions including a contact intermediate layer composed of a conductive oxide between said cathode and the lines.
According to still another aspect of the present invention, in the above described electroluminescence display device, the contact intermediate layer is composed of the same material as the other electrode of the first and second electrodes, i.e., the lower electrode (or an anode).
Further, according to another aspect of the present invention, a metal material is used for the lines at the terminal side.
Conventionally, a metal material such as Al is often used for the lines, for example, the lines extending from the terminal side. However, oxide is likely to be generated on the surface of such a metal material as Al. According to the present invention, a contact intermediate layer composed of a conductive oxide is provided between the lines in the contact portions (the first and second contact portions) which connect the lines extending from the terminal side and the lines extending from the upper layer electrode or the cathode. It is therefore possible to prevent the line material from being oxidized in the process of forming the contacts. Further, because increase in the resistance can be prevented by oxidation of the contact intermediate layer itself, it is possible to ensure low contact resistance.